Dissection of printed edges from a fabrication layout for correcting proximity effects

ABSTRACT

Techniques for fabricating a device include forming a fabrication layout, such as a mask layout, for a physical design layer, such as a design for an integrated circuit, and identifying evaluation points on an edge of a polygon corresponding to the design layer for correcting proximity effects. Techniques include correcting for proximity effects associated with an edge in a first fabrication layout by determining whether any portion of the edge corresponds to a target edge in a design layer. The first fabrication layout corresponds to the design layer that indicates target edges for a printed features layer. If any portion of the edge corresponds to the target edge, then it is determined whether to establish an evaluation point on the edge. Then it is determined how to correct the edge for proximity effects based on the evaluation point. In case it is determined that no portion of the edge corresponds to the target edge, then no evaluation point is selected on the edge.

RELATED APPLICATIONS

[0001] The present application is a divisional of commonly ownedco-pending U.S. patent application Ser. No. 09/676,375, “DISSECTION OFPRINTED EDGES FROM A FABRICATION LAYOUT FOR CORRECTING PROXIMITYEFFECTS” filed Sep. 29, 2000 by Christophe Pierrat and Youping Zhang.

[0002] This application is related to U.S. patent application Ser. No.09/675,582 (Attorney Docket 50275-0013), entitled “Dissection Of CornersIn A Fabrication Layout For Correcting Proximity Effects,” filed on Sep.29, 2000, invented by Christophe Pierrat and Youping Zhang.

[0003] This application is related to U.S. patent application Ser. No.09/675,197 (Attorney Docket 50275-0014), entitled “Dissection Of EdgesWith Projection Points In A Fabrication Layout For Correcting ProximityEffects,” filed on Sep. 29, 2000, invented by Christophe Pierrat andYouping Zhang.

[0004] This application is related to U.S. patent application Ser. No.09/676,356 (Attorney Docket 50275-0016), entitled “Selection OfEvaluation Point Locations Based on Proximity Effects Model AmplitudesFor Correcting Proximity Effects In A Fabrication Layout,” filed on Sep.29, 2000, invented by Christophe Pierrat and Youping Zhang.

[0005] This application is related to U.S. patent application Ser. No.09/728,885 (Attorney Docket 50275-0017), entitled “Displacing EdgeSegments On A Fabrication Layout Based on Proximity Effects ModelAmplitudes For Correcting Proximity Effects,” filed on Dec. 1, 2000,invented by Christophe Pierrat and Youping Zhang.

[0006] This application is related to U.S. patent application Ser. No.09/130,996, entitled “Visual Inspection and Verification System,” filedon Aug. 7, 1998.

[0007] This application is related to U.S. patent application Ser. No.09/153,783, entitled “Design Rule Checking System and Method,” filed onSep. 16, 1998.

[0008] This application is related to U.S. patent application Ser. No.09/544,798, entitled “Method and Apparatus for a Network Based MaskDefect Printability Analysis System,” filed on Apr. 7, 2000.

[0009] This application is related to U.S. patent application Ser. No.09/154,415, entitled “Data Hierarchy Layout Correction and VerificationMethod and Apparatus,” filed on Sep. 16, 1998.

[0010] This application is related to U.S. patent application Ser. No.09/154,397, entitled “Method and Apparatus for Data HierarchyMaintenance in a System for Mask Description,” filed on Sep. 16, 1998.

[0011] This application is related to U.S. patent application Ser. No.09/632,080, entitled “General Purpose Shape-Based Layout ProcessingScheme for IC Layout Modifications,” filed on Aug. 2, 2000.

BACKGROUND OF THE INVENTION

[0012] 1. The Field of the Invention

[0013] This invention relates to the field of printed featuremanufacturing, such as integrated circuit manufacturing. In particular,this invention relates to automatically identifying evaluation pointswhere errors are computed and analyzed to achieve improved agreementbetween a design layout and an actual printed feature.

[0014] 2. Description of Related Art

[0015] To fabricate an integrated circuit (IC), engineers first use alogical electronic design automation (EDA) tool, also called afunctional EDA tool, to create a schematic design, such as a schematiccircuit design consisting of symbols representing individual devicescoupled together to perform a certain function or set of functions. Suchtools are available from CADENCE DESIGN SYSTEMS and from SYNOPSYS. Theschematic design must be translated into a representation of the actualphysical arrangement of materials upon completion, called a designlayout. The design layout uses a physical EDA tool, such as thoseavailable from CADENCE and AVANT!. If materials must be arranged inmultiple layers, as is typical for an IC, the design layout includesseveral design layers.

[0016] After the arrangement of materials by layer is designed, afabrication process is used to actually form material on each layer.That process includes a photo-lithographic process using a mask havingopaque and transparent regions that causes light to fall onphotosensitive material in a desired pattern. After light is shinedthrough the mask onto the photosensitive material, the light-sensitivematerial is subjected to a developing process to remove those portionsexposed to light (or, alternatively, remove those portions not exposedto light). Etching, deposition, diffusion, or some other materialaltering process is then performed on the patterned layer until aparticular material is formed with the desired pattern in the particularlayer. The result of the process is some arrangement of material in eachof one or more layers, here called printed features layers.

[0017] Because of the characteristics of light in photolithographicequipment, and because of the properties of the material alteringprocesses employed, the pattern of transparent and opaque areas on themask is not the same as the pattern of materials on the printed layer. Amask design process is used, therefore, after the physical EDA processand before the fabrication process, to generate one or more mask layoutsthat differ from the design layers. When formed into one or more masksand used in a set of photolithographic processes and material alteringprocesses, these mask layouts produce a printed features layer as closeas possible to the design layer.

[0018] The particular size of a feature that a design calls for is thefeature's critical dimension. The resolution for the fabrication processcorresponds to the minimum sized feature that the photolithographicprocess and the material processes can repeatably form on a substrate,such as a silicon wafer. As the critical dimensions of the features onthe design layers become smaller and approach the resolution of thefabrication process, the consistency between the mask and the printedfeatures layer is significantly reduced. Specifically, it is observedthat differences in the pattern of printed features from the mask dependupon the size and shape of the features on the mask and the proximity ofthe features to one another on the mask. Such differences are calledproximity effects.

[0019] Some causes of proximity effects are optical proximity effects,such as diffraction of light through the apertures of the opticalsystems and the patterns of circuits that resemble optical gratings.Optical proximity effects also include underexposure of concave corners(inside corners with interior angles greater than 180 degrees) andoverexposure of convex corners (outside corners with interior anglesless than 180 degrees), where the polygon represents opaque regions, anddifferent exposures of small features compared to large featuresprojected from the same mask. Other causes of proximity effects arenon-optical proximity effects, such as sensitivity of feature size andshape to angle of attack from etching plasmas or deposition bysputtering during the material altering processes, which cause featuresto have shapes and sizes that have decayed from or accumulated ontotheir designed shapes and sizes.

[0020] In attempts to compensate for proximity effects, the mask layoutscan be modified. To illustrate, FIG. 1A shows mask items 170, such aswindows or opaque areas on a mask, represented by edges 171 on one ormore polygons, and some printed features 180 with exaggerated proximityeffects. FIG. 1A does not represent an actual example, but is providedsimply to illustrate the concepts of proximity effects and maskmodifications to attempt to correct for proximity effects. To makeapparent the illustrated proximity effects, the projection of theoriginal polygons 171 is shown as a fine line around the printedfeatures 180. Note that the printed features 180 includes a spuriousconnection feature 182, an edge 185 entirely displaced inside thecorresponding edge of the original outline 171, and an end line 183completely inside the outline of the original items 171.

[0021]FIG. 1B illustrates various ways mask items 190 are modified tocorrect for such effects. FIG. 1B is not a particular example of aparticular set of corrections that actually mitigate the proximityeffects illustrated in FIG 1A. The corrections available includehammerheads 192 (i.e. hammerheads 192 a and 192 b) added to ends ofitems to compensate for overexposure of the entire end line of afeature. Also shown are biases 196 (i.e. biases 196 a and 196 b) appliedalong portions of a straight edge of a feature. A negative bias like 196represents a portion of an opaque area made transparent (or a portion ofa window made opaque). In this case the negative bias reduces the sizeof the item on a mask to avoid generating the spurious feature 182 ofFIG. 1A. Also shown are assist features 194 (i.e. assist features 194 aand 194 b), which are separate items smaller than the resolution of thephotolithographic process and thus too small to be formed in aphotoresist layer, but which are sufficiently large to effectdiffraction patterns that influence larger nearby features. The assistfeatures 194 are intended to move the edge 185 of the printed features180 in FIG. 1A toward the outline 171 of the original mask items 170.Also shown is a subresolution serif 193 of extra opaque material tocompensate for overexposure at convex corners of opaque areas, and ananti-serif 197 indicating where opaque material, if any, is removed tocompensate for underexposure at concave corners of opaque polygons.These corrections are listed to illustrate the concepts of correcting amask to compensate for proximity effects. The illustrated corrections donot necessarily correct the depicted features.

[0022] In rule-based proximity corrections, corrections such as theserifs and biases of a predetermined size are automatically placed atcorners and edges of fabrication layout shapes like the mask shapes 170.Other rules may include adding assist shapes to a mask near desiredfeatures from the design layout and adding hammerheads to short endlines of the desired features. Experience of engineers accumulatedthrough trial and error can be expressed as rules, and applied duringthe fabrication design process. For example, a rule may be expressed asfollows: if a feature is isolated, then widen the opaque region in themask by a particular amount to compensate for expected overexposure, sothat the feature prints properly.

[0023] The experience captured in rule-based corrections is garnered bygoing through the fabrication process repeatedly with different masklayouts, making adjustments and observing the results. However, thisprocess consumes time and manufacturing capacity. Even if suchexperimental, rule-forming runs are made, the resulting rules often donot give satisfactory results as the features become more complicated,or become smaller, or interact in more confined areas, or involve anycombination of these effects.

[0024] Because rule based corrections are often imprecise and timeconsuming to revise and test, a proximity effects model is oftendeveloped and used to predict the effects of a change in the mask layoutwith more precision and with fewer off line fabrication runs.

[0025] A proximity effects model is typically built for a particularsuite of equipment and equipment settings assembled to perform thefabrication process. The model is often built by performing thefabrication process one or a few times with test patterns on one or moremask layouts, observing the actual features printed (for example with ascanning electron micrograph), and fitting a set of equations or matrixtransformations that most nearly reproduce the locations of edges offeatures actually printed as output when the test pattern is provided asinput. The output of the proximity effects model is often expressed asan optical intensity. Other proximity effects models provide acalibrated output that includes a variable threshold as well as anoptical intensity. Some proximity effects models provide a calibratedoutput which indicates a printed edge location relative to a particularoptical intensity as a spatial deviation. Thus some model outputs cantake on values that vary practically continuously between some minimumvalue and some maximum value.

[0026] Once a proximity effects model is produced, it can be used in thefabrication design process. For example, the proximity effects model isrun with a proposed mask layout to produce a predicted printed featureslayer. The predicted printed features layer is compared to the designlayer and the differences are analyzed. Based on the differences,modifications to the proposed mask layout are made, such as by adding orremoving serifs 193, or adding or removing anti-serifs 197. After makingthese corrections to the proposed mask layout, a final mask layout isgenerated that is used in the fabrication process.

[0027] A proximity effects model typically produces predictions thatlead to corrections that are more accurate than rule based corrections.However, a proximity effects model consumes computational resources byan amount that is related to the number of points of interest whereamplitudes are computed. For typical mask layouts, the number of pointswhere the model could be run is large, and the computation time isprohibitive. Therefore it is typical to run the proximity effects modelat selected evaluation points located on the edges of features, todetermine the correction needed, if any, at each evaluation point, andthen to apply that correction to all the points on a segment of the edgein the vicinity of the evaluation point.

[0028] If too many evaluation points are selected, the model runs tooslowly and the correction procedure takes too long. If too fewevaluation points are selected, the model may not detect criticallocations where corrections are necessary. Another undesirable effect oftoo few evaluation points is that segments can become too large, so thateven if a correction is accurate for the evaluation point, thecorrection is applied to too much of the edge. This causes undesirablechanges on the edge away from the evaluation point.

[0029] What are needed are new ways to select evaluation points anddefine their vicinity for the mask layouts, so that a reasonable numberof comparisons can be made and so that effective modifications to themask layouts can be suggested.

SUMMARY OF THE INVENTION

[0030] According to techniques of the present invention, edges ofpolygons in a mask layout are dissected into segments defined bydissection points, with each segment having an evaluation point,according to modified dissection rules. These techniques allow thespacing of evaluation points and dissection points to be automaticallyadapted to portions of each edge where changes in the proposed masklayout are most likely needed. According to these techniques, dissectionpoints are closer together where proximity effects are more significantand are farther apart where proximity effects are less significant. Thusunnecessary evaluation points are eliminated and the analysis process isspeeded up, while still retaining needed evaluation points.

[0031] In one aspect of these techniques, all dissection points can bederived from just a few easily defined parameters. The first parameteris a corner dissection segment length, Lcor, used as a target length tocompute dissection points for corner segments that include at least onevertex of the polygon when the length of the edge allows. The secondparameter is a detail dissection segment length, Ldet, used as a targetlength to derive dissection points for segments along portions of anedge where changes in proximity effects are expected to be significant,such as adjacent to corner segments and around projection points whenthe length of the edge allows. A projection point is a location on anedge where a vertex of another edge comes close enough to introducesharp changes in the proximity effects. A halo distance is a parameterthat determines whether a vertex is close enough to introduce animportant effect. A maximum dissection segment length, Lmax, bydefinition larger than either Lcor or Ldet, is used as a target lengthto further dissect long portions of the edge that would otherwise not bedissected, such as away from corners and projections points. Extraprecision is achieved if a true-gate extension length parameter Lext isused on edges of shifters forming true-gates. The dissection parametersLcor, Ldet, Lmax, halo and Lext are pre-set and accessed as needed.

[0032] In one aspect of the disclosed techniques, a profile ofamplitudes from a proximity effects model along an edge of a feature ina design layout is used to dissect the edge. This amplitude profilereflects the level of proximity effects for the given layout andfabrication process. By using this information, a better dissection ofthe edge is achieved in which the segment length between successivedissection points depends on the magnitude of the proximity effects. Forexample, on a portion of an edge where the magnitude of the proximityeffects is larger than another portion, the segment length is madelonger. This kind of dissection avoids making the segment length tooshort. A segment that is too short, for example in a corner, may resultin a very large correction, and may prevent the overall corrections forthe layout from converging.

[0033] In another aspect, the dissection parameters are derived from thesegment lengths based on profiles of amplitudes. According to themodified dissection rules, each evaluation point is placed along asegment at a predetermined fraction of the distance from one dissectionpoint to the other defining each segment, where the predeterminedfraction depends on the type of segment. For example, an evaluationpoint is placed closer to the non-vertex dissection point in a cornersegment; and is placed midway between dissection points in other typesof segments. Also according to the modified dissection rules, segmentsare located on polygon edges according to the type of edge, such aswhether the edge is a line end, a turn end, or a longer edge. These edgetypes are deduced from the length of the edge compared to the dissectionparameters and whether one or both vertices of the edge are concave orconvex corners.

[0034] Specifically, according to some aspects of the invention,techniques for correcting for proximity effects associated with an edgein a first fabrication layout include determining whether any portion ofthe edge corresponds to a target edge in a design layer. The firstfabrication layout corresponds to the design layer that indicates targetedges for a printed features layer. If any portion of the edgecorresponds to the target edge, then it is determined whether toestablish an evaluation point on the edge. Then it is determined how tocorrect the edge for proximity effects based on the evaluation point. Inone embodiment of this aspect, if it is determined that no portion ofthe edge corresponds to the target edge, then no evaluation point isselected on the edge.

[0035] According to other aspects, techniques for correcting proximityeffects associated with an edge in a first fabrication layout includesdetermining whether a first part of the edge is less important in thedesign layer than a second part of the edge. The first fabricationlayout corresponds to a design layer that indicates target edges for aprinted features layer. If it is determined that the first part is lessimportant than the second part, then an evaluation point is establishedon the second part. In this case, no evaluation point is established onthe first part. It is then determined how to correct at least a portionof the edge for proximity effects based on an analysis at the evaluationpoint.

[0036] According to one embodiment of this aspect of the invention, thefirst part is determined to be less important than the second part ifthe first part does not correspond to a target edge in the design layerand the second part does correspond to a target edge in the designlayer.

[0037] According to another embodiment of this aspect of the invention,the first part is determined to be less important than the second partif the first part does not correspond to a true-gate in the design layerfor an integrated circuit and the second part does correspond to atrue-gate in the design layer.

[0038] According to another embodiment of this aspect of the invention,if the second part corresponds to an edge of a true-gate and a true-gateextension parameter is defined, then, before establishing an evaluationpoint, the second part is extended to displace the first part. Thedisplacement is by the smaller amount of a length of the first part andthe true-gate extension parameter.

[0039] According to several embodiments, the edge is split into segmentssuch that no segment on the edge has a length less than a minimum targetlength.

[0040] According to a different embodiment, a computer system forcorrecting proximity effects associated with an edge in a firstfabrication layout includes a computer readable medium. The firstfabrication layout corresponds to a design layer that indicates targetedges for a printed features layer. The computer readable medium carriesdata representing the edge and at least a portion of the design layout.One or more processors are coupled to the computer readable medium. Theprocessors are configured for determining whether a first part of theedge is less important in the design layer than a second part of theedge. If it is determined that the first part is less important than thesecond part, then an evaluation point is established on the second part,and no evaluation point is established on the first part. A proximityeffects model is run for the evaluation point to produce a modelamplitude. A correction is determined based on an analysis of the modelamplitudes. The correction is then applied to at least some of the edge.

[0041] In another embodiment, a mask for fabricating a printed featureslayer includes an opaque region having an edge corrected for proximityeffects. The edge corresponds to a target edge in a design layer. Thedesign layer indicates target edges for a printed features layer. Theedge is displaced from the corresponding target edge by a correctiondistance. The correction distance is based on analysis of an amplitudeoutput by a proximity effects model at an evaluation point on thecorresponding target edge. The evaluation point is based on determiningwhether a first part of the edge is less important in the design layerthan a second part of the edge. If it is determined that the first partis less important than the second part, then an evaluation point isestablished on the second part but not on the first part.

[0042] According to other aspects, techniques for correcting proximityeffects associated with an edge in a first fabrication layout includesdetermining whether a first part of the edge is less important in thedesign layer than a second part of the edge. The first fabricationlayout corresponds to a design layer that indicates target edges for aprinted features layer. If it is determined that the first part is lessimportant than the second part, then a first initial segment isestablished on the first part extending from a junction to the oppositeend of the first part. The first part and the second part meet at thejunction. A second initial segment is established on the second partextending from the junction to the opposite end of the second part. Anevaluation point is established on the second part. Then it isdetermined how to correct at least a portion of the edge for proximityeffects based on an analysis at the evaluation point.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The present invention is illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings and inwhich like reference numerals refer to similar elements and in which:

[0044]FIG. 1A is a plan view of two items on a mask and the resultingprinted features showing proximity effects.

[0045]FIG. 1B is a plan view of elements on a modified mask afteradjusting the mask of FIG. 1A in an attempt to correct for proximityeffects.

[0046]FIG. 2 is a flow chart showing the sequence of processes andlayouts utilized in the formation of devices having printed featureslayers according to one embodiment of the disclosed techniques.

[0047]FIG. 3 is a flow chart of a process for selecting dissectionpoints according to one embodiment.

[0048]FIG. 4A is a plan view of elements on a mask used as input to anillustrative proximity effects model.

[0049]FIG. 4B is a plan view illustrating printed features withproximity effects that might result from the mask items of FIG. 4Asuperposed on an ideal projection of the mask items.

[0050]FIG. 4C is a graph of contours of constant model amplitude outputon a two dimensional array resulting from the input of FIG. 4A for theexemplary proximity effects model.

[0051]FIG. 4D is a graph of a profile of model amplitude along a lineshown in FIG. 4C for the exemplary proximity effects model.

[0052]FIG. 5A is a plan view of elements on a second mask layout used asinput to a second proximity effects model.

[0053]FIG. 5B is a graph of a profile of model amplitude along an edgeof a polygon from FIG. 5A.

[0054]FIG. 5C is a graph of a first derivative of the profile of FIG. 6Bshowing dissection points according to one embodiment.

[0055]FIG. 5D is a graph of a second derivative of the profile of FIG.6B showing dissection points according to another embodiment.

[0056]FIG. 6 shows an exemplary test pattern for a fabrication process,a profile location, and the corresponding exemplary model output alongthe profile.

[0057]FIG. 7 is a flow diagram for setting dissection parametersaccording to one embodiment.

[0058] FIGS. 8A-8E are partial plan views of polygons from mask layoutsshowing placement of dissection points and evaluation points accordingto one embodiment.

[0059]FIGS. 9A and 9B are flow diagrams for selecting dissection pointsusing the dissection parameters according to one embodiment.

[0060]FIGS. 10A through 10C are partial plan views of overlappingpolygons from two mask layouts, showing the placement of dissectionpoints and evaluation points when not all of a polygon is printed,according to several embodiments.

[0061]FIG. 11 is a flow diagram of a process employed when changingedges for selecting dissection points and evaluation points according toone embodiment.

[0062]FIG. 12 is a block model of a computer system configured accordingto one embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0063] Techniques are described for correcting fabrication layouts forproximity effects during the fabrication of printed features layers,such as in integrated circuits.

[0064] Functional Overview

[0065]FIG. 2 shows the processes and layouts produced according to thepresent techniques for designing and fabricating printed features layers249 for devices 250. The conventional processes are modified to includenew techniques for selecting evaluation and dissection points, asrepresented in FIG. 2 by process 260.

[0066] The conventional processes include the Functional EDA process 210that produces the schematic diagram 215. The physical EDA process 220converts the schematic diagram to a design layout made up of one or moredesign layers 225 (e.g. design layers 225 a, 225 b, and 225 c). Aftermask layouts 235 (e.g. mask layouts 235 a, 235 b, and 235 c) areproduced, the conventional processes employ a fabrication process 240 toproduce the printed features layer 249. The printed features layer 249may be a layer in a printed circuit or the mask used to produce thelayer in the printed circuit. In the former case, the fabricationprocess includes one process 243 for forming the mask and a secondprocess 245 to produce the layer of the integrated circuit using themask. If the printed features layer 249 is the mask, step 245 isskipped.

[0067] According to techniques of the present invention, the fabricationdesign process 230 includes a new process 260 for obtaining dissectionand evaluation points. In one embodiment, these new dissection andevaluation points are determined for a proposed mask layout 231 and usedwith a proximity effects model in step 232 to produce a model output233. The model output is related to the predicted location of edges inthe printed features layer if a printed features layer were fabricatedwith a mask having the proposed mask layout. Hence, in FIG. 2, the modeloutput is called the predicted printed features edge.

[0068] Each evaluation point is established for a segment of an edgefrom the proposed mask layout. The segment is defined by a pair ofdissection points associated with the evaluation point. Once thedissection points and evaluation points are selected, the proximityeffects model is run at the evaluation points. In step 234, thepredicted printed features edge 233 for the evaluation point is comparedto a target position for that edge in the design layer. Based on ananalysis of the comparison, a correction distance is computed. Thecorrection distance is then applied to the entire segment betweendissection points associated with the evaluation point on the edge ofthe affected polygon. As each correction is applied, the mask layout isadjusted in step 236. As a result of the adjustments in step 236 thepolygons on the adjusted mask layout may differ from the polygons in theproposed mask layout, as well as from the polygons in the design layers225. The final mask layouts 235 are then based on the adjusted masklayouts produced by the adjustment process 236.

[0069] Obtaining Dissection and Evaluation Points

[0070] An overview of one embodiment for the new dissection/evaluationpoints selection process 260 is illustrated in FIG. 3. According to thisembodiment, a few dissection parameters are established in response topredictions from a proximity effects model in step 310. In anotherembodiment, a human operator sets the dissection parameters. Those fewdissection parameters represent the necessity of dissection alongvarying portions of various types of edges. These parameters are used toselect all the dissection points and evaluation points on an edge of apolygon of a fabrication layout in step 330. In step 370, an evaluationpoint is placed on each segment defined by a pair of successivedissection points on the edge. The process then continues over all theedges needing dissection, on all the polygons in the layout.

[0071] For example, in step 390, it is determined whether the currentedge is the last edge on the current polygon. If not, process flowpasses to step 392 where the next edge requiring dissection on thepolygon is selected and made the current edge. Flow then returns to step330 in which the edge newly made current is dissected according to thedissection parameters. If the edge just processed by steps 330 and 370is the last edge of the current polygon, process flow passes to step 394in which it is determined whether the current polygon is the last oneneeding dissection in the fabrication layout. If not, the next polygonis made the current polygon in step 396, and control passes to step 392in which the first edge needing dissection on the polygon is made thecurrent edge. If the current polygon is the last on the layout then flowpasses to step 232 of FIG. 2, where the proximity effects model is runat the selected evaluation points.

[0072] In an alternative embodiment, step 394 includes making anotherfabrication layout the current fabrication layout and then passingcontrol to step 396. Such a subsequent layout may be necessary todissect; for example, when the two fabrication layouts are used todoubly expose the same printed features layer, so that edges from thesecond fabrication layer become part of the printed features layer. Thiscircumstance will be described in more detail later below.

[0073] Examples of Selecting Dissection/Evaluation Points & SettingDissection Parameters using a Proximity Effects Model

[0074] According to one embodiment, the proximity effects model is usedto set the dissection parameters, for example, in step 310. Theseparameters remain constant during the dissection of several polygons instep 330, on one or more proposed fabrication layouts during thefabrication layout design process, step 230.

[0075] Herein the term amplitude is used to describe the proximityeffects model output associated with a location no matter what model isused. In general, the models are built to accept input representingpolygons of opaque or transparent material. In general, an amplitudevalue is then produced for each location of interest. A thresholdamplitude between the minimum amplitude value and the maximum amplitudevalue can be associated with the model. As used here, the thresholdamplitude is associated with the location where the edge of a feature isactually printed. For example, where spatial deviation is output by themodel, a threshold of zero marks the location of an edge. As anotherexample, where the model output is simple optical intensity, a non-zeropositive value associated with the printed edge serves as the threshold.In some models, the threshold value associated with an edge may itselfbe a function of several variables such as the optical intensity andintensity gradient at a location.

[0076] To understand how dissection parameters can be derived fromproximity effects model output, consider FIGS. 4 and 5.

[0077]FIG. 4A shows two items 410 and 420 represented by polygons 411and 421, respectively, in a mask layout provided as input to a proximityeffects model that has been developed for a particular suite ofequipment and settings. FIG. 4B shows the printed features 412 and 422,with shapes 413 and 423, respectively, that would be produced from amask formed from the mask layout of FIG. 4A according to the proximityeffects model.

[0078] For reference, FIG. 4B also shows the polygons 411 and 421 ifthose were projected perfectly onto the printed features layer. Thedifferences between the ideal and actually printed features are due toproximity effects. For example, arrow 415 shows the deviation of printedfeature 413 from the design layout polygon 411 at one location on thepolygon 411. Similarly, arrow 425 shows the deviation of printed feature423 from the design layout polygon 421 at a location on that polygon421. These deviations are output by one kind of proximity effects model.

[0079]FIG. 4C illustrates contours 450 a-450 g of constant amplitude ona two dimensional array of model output for one kind of proximityeffects model, if it were to be run for every point on such an array.One particular contour representing the threshold value, contour 450 c,is shown as a bold line. The shapes formed by the bold contour 450 crepresent this model's prediction of the shape of the printed featuresaccounting for all proximity effects in the system. As can be seen bythe bold contour 450 c in FIG. 4C, this proximity effects model predictsprinted features with shapes that agree with those that actually wouldbe produced on the printed features layer, as shown in FIG. 4B.

[0080] For simplicity of explanation, the proximity effects model thatoutputs contours like those shown in FIG. 4C is used as an exemplaryproximity effects model in the rest of this specification, unlessotherwise stated. The techniques of the present invention, however, workwith any proximity effects model, including those models which have beencalibrated to produce amplitudes representing the predicted printedfeature

[0081]FIG. 4C also shows a double arrow 460 along which a profile ofmodel amplitude is plotted in FIG. 4D. In FIG. 4D, model amplitude isexpressed in arbitrary units on the vertical axis 484 from zero to amaximum represented by a value of 1.0. Distance along the double arrowof FIG. 4C from left to right is shown on the horizontal axis 482 ofFIG. 4D, in arbitrary distance units. Along this profile the modelamplitude begins at zero, rises steadily from about 3 distance units toabout 5 distance units, flattens out to about 7 units, then dips atabout 8 units, rises to a peak at about 9 distance units, and then fallsoff to zero again at about 13 distance units.

[0082] The threshold value 470 of about 0.3 model amplitude unitscorresponds to the value of contour 450 c. Where the model output 465 isabove the threshold value 470, a feature is predicted to be printed inthe printed features layer; where the model output 465 is below thethreshold value 470, no feature is predicted to be printed. Where themodel output 465 equals the threshold value is where the edge of theprinted feature is predicted to lie. Of course model output units arearbitrary and could be inversely related to what is printed, so that lowamplitudes represent printed features and high amplitudes represent noprinting. In such a case, the threshold would mark the value below whicha feature is printed.

[0083] The selection of dissection points and the derivation ofdissection parameters from a profile of model output is illustrated inFIGS. 5A, 5B, 5C and 5D.

[0084]FIG. 5A is a plan view of elements on a second mask layout used asinput to another proximity effects model. The mask elements include twotransparent items, 510 and 520, represented by a first polygon withedges 511 through 516 and a second polygon with edges 521 through 524,respectively. On the polygon of item 510, points 532 and 533 indicatewhere proximity effects are expected to vary sharply because of thecorner point 538 between edges 514 and 515. Also shown are points 531,534, 535 and 536 on the polygon of item 510 where proximity effects areexpected due to the corner between edges 521 and 522 of the polygon ofitem 520. Points 531 through 536 are projection points, where proximityeffects on one edge are expected to vary sharply because of theproximity of vertices of other edges.

[0085] A profile of model amplitudes along one of these edges reflectsthe size of proximity effects on that edge for this particular layoutand model (where the model represents this particular suite offabrication equipment and settings). By using information in thisprofile of amplitudes, a better dissection of this edge can be performedin which the segment length depends on the proximity effect magnitude.

[0086]FIG. 5B is a graph of a profile 540 of model amplitude along edge511 of the polygon of item 510 shown in FIG. 5A, from its vertex incommon with edge 516 to its vertex in common with edge 512. For purposesof illustration, the threshold for this model is taken as 0.23. Thismodel then predicts that this edge 511 will print from about 100nanometers (nm) to about 1100 nm. Arrow 541 indicates the location ofprojection point 531 and the value of amplitude at that projectionpoint. Curve 543 indicates where the amplitude curve 540 would beexpected to lie if there were no variation in proximity effects on edge511 due to item 520. Arrow 542 indicates the location of projectionpoint 532 and its amplitude. Curve 544 indicates where the amplitudeprofile 540 would be expected to lie if there were no variation inproximity effects on edge 511 due to corner point 538. As expected, theproximity effects represented by curve 540 vary in the vicinity of theprojection points.

[0087] This profile 540 is used to illustrate how particular dissectionpoints are selected on this edge, as well as how the dissectionparameters used for dissecting other edges are derived from model outputon this edge, according to this embodiment. In other embodiments,variations on the techniques described here are used.

[0088] This profile 540 illustrates a length scale of proximity effectsat corners of a polygon. That corner length scale is found from a vertexof the edge to a position where the profile flattens near amplitude 0.3.That length scale is about 200 nm. In an alternative embodiment, thecorner length scale is selected to be the distance to the thresholdvalue of 0.23, which is about 150 nm. In still another embodiment, thecorner length scale is taken to be a distance to an amplitude that is agiven percentage of the threshold value. In this case, the givenpercentage is selected based on how aggressive a correction is sought inthe vicinity of corners. Where more aggressive correction is desired,the percentage is chosen to be less than 100%. Where less aggressivecorrection is desired, the percentage is chosen to be greater than 100%.In still other embodiments, the corner length scale is associated withtypes of corners, e.g., concave or convex, or angle at the corner, orboth. The corner length scale so derived provides a target length to usewhen dissecting corners where the edge allows.

[0089] This profile also suggests a length scale for details nearprojection points. Considering both sides of each projection point, thatdetail length scale should be about half the distance over which curve540 deviates from curves 543 and 544. That detail length scale is about100 nm. The detail length scale so derived provides a target length touse when dissecting details where the edge allows. This profile alsosuggests a maximum length scale where the curve 540 hardly varies atall, between the two projection points, of about 300 nm. The maximumlength scale so derived provides a target length to use when dissectingedges away from corners and details, where the edge allows.

[0090] Along other edges, the distances for the amplitude curve toflatten, the distances over which projection points have noticeableeffects, and the length of the undisturbed sections are expected tovary. Thus, dissection parameters based on these must be derived fromthe observed distributions of length scales. In a preferred embodiment,instead of using a single corner scale from one polygon of one testlayout, a statistical corner scale, such as an average corner scale, ormedian corner scale, is used as a dissection parameter near corners. Thestatistical corner scale is derived from the corner segment lengths ofseveral polygons on the same test layout or on several layouts. In otherembodiments, different corner scales are derived for different cornertypes, e.g., concave or convex, or different ranges of angles or both.For example, corner segments associated with acute angles less than 45degrees can be averaged separately from corner segments associated withintermediate angles from 45 through 80 degrees which themselves areaveraged separately from corner segments associated with nearly rightangles from 80 to 100 degrees.

[0091] In another model where proximity effects are different, thevalues of the length scales and their distributions are also expected tobe different. For example, in an equipment suite with lower resolutionoptics and material processes, the length parameters are expected to belarger. That is, a distance from a vertex of an edge to a flatteningpoint is expected to be greater for all edges tested, and, therefore,the derived dissection parameter is expected to be larger.

[0092] In one embodiment, an evaluation point for a corner segment isdetermined on this curve. According to this embodiment, the evaluationpoint is placed where the curve has a value equal to a given percentageof the amplitude value where the curve first flattens. For example, ifthe given percentage is set at 75%, then because curve 540 flattens atabout 0.31 amplitude units, the evaluation point is selected where thecurve has a value of 75% of 0.31, i.e., a value of 0.23. This occurs ata distance of about 100 nm. In another embodiment, the evaluation pointis placed where the profile has a value equal to a given percentage ofthe threshold value, e.g., 90% of the threshold value of 0.23, which is0.21. This occurs at about 90 nm on curve 543.

[0093] The following paragraphs define particular embodiments fordetermining dissection points and length scales for dissectionparameters. Other embodiments rely on variations of these techniquesknown in the art.

[0094]FIG. 5C is a graph of a curve 550 representing a first derivativeof the profile 540 of FIG. 5B. The first derivative is a measure of theslope of the profile 540 at each point along the profile. Where theprofile 540 flattens the slope 550 approaches zero. A particular smallvalue of slope, say 0.001, is chosen to indicate where a profile isflat. A profile is flat where the absolute value of its first derivativeis less than this particular value. Starting from a distance of 0 nm,the curve 550 first becomes less than this particular value at about 200nm. According to this embodiment, a corner segment is placed on thisedge from 0 nm to 200 nm where the first derivative decreases to theparticular value. The corner segment is marked by dissection pointslocated at the vertex at 0 nm and at about 200 nm.

[0095] The 200 nm length of this corner segment is then submitted to aprocess to derive a corner dissection segment length parameter, Lcor.That process performs a weighted average of this corner length for thisedge with other corner lengths from this and other edges to derive thecorner dissection segment length, Lcor. In one embodiment, the weightsare all the same.

[0096] Similarly, moving along edge 511 from its vertex in common withedge 512, at a distance of 1200 nm, the slope first increases above−0.001 at about 1000 nm. That is, the absolute value of the firstderivative decreases below the particular value of 0.001 at about 1000nm. This indicates a corner segment for this edge lies from distance1000 nm to 1200 nm, and dissection points are placed at both thoselocations. As above, the 200 nm corner segment length is submitted tothe process to derive the corner dissection segment length dissectionparameter, Lcor.

[0097]FIG. 5C also shows arrows 551 and 552, which indicate thelocations of projection points 531 and 532, respectively. Though theabsolute value of slope at 551 may exceed the particular value of 0.001,this does not appear to be the case at 552. The slope near theprojection point at 552 appears to be affected by the opposite slopeassociated with the corner segment. Along curve 550, the absolute valueof the slope exceeds the particular value of 0.001 also near pointsindicated by arrows 553 and 554, at about 300 nm and 860 nm,respectively.

[0098] According to this embodiment, dissection points are added at eachlocation where the absolute value of slope reaches a local maximum abovethe small particular value. One embodiment adds dissection points at553, 551 and 554, which are located at about 300 nm, 480 nm and 860 nm,respectively. Thus, segments that extend from 200 to 300 nm, 300 to 480nm, and 860 to 1000 nm are added to the two corner segments alreadydefined for this particular edge 511. A residual edge that undergoesfurther dissection remains between points 551 and 554, from about 480 to860 nm. Evaluation points are then placed on this edge, one evaluationpoint on each segment. In this embodiment, each evaluation point isplaced midway between the dissection points on each non-corner segment.

[0099] The non-corner segment lengths obtained from these embodimentsare sent to a process to derive other dissection parameters, such as adetail dissection segment length, Ldet, and a maximum dissection segmentlength, Lmax. In one embodiment of the process to derive dissectionparameters, a histogram of segment lengths is formed and the detaildissection segment length, Ldet, is derived from a mode of thedistribution associated with the smallest lengths. In this embodiment, amaximum dissection segment length is derived from a second modeassociated with the next smallest lengths. In other embodiments, lengthscales may be derived from such a distribution of lengths using anymethods known in the art. Then, dissection parameters are set based onthe derived length scales.

[0100] In another embodiment of selecting dissection points from modeloutput along a particular edge, dissection points are also added betweenthese local slope maxima, either at a midpoint, or where the slopes areinterpolated to closest approach zero. This embodiment will tend toproduce twice the number of segments and cause the dissection parametersderived from the resulting segments to have about half the size.Evaluation points are then placed at the midpoints of the resultingsegments.

[0101] In still another embodiment, dissection points are added atnon-adjacent points where the slope curve 550 has a slope whose absolutevalue equals the particular minimum slope value, e.g., 0.001. Thisembodiment adds two dissection points for each local maximum such as at553 and 554, one on either side of each local maximum.

[0102] In another embodiment, before adding dissection points, thesecond derivative of the profile 540 is computed and examined. FIG. 5Dis a graph of a curve 560 representing a second derivative of theprofile 540 of FIG. 5B. The second derivative measures the curvature ofa profile. A positive value indicates that the profile is turning abouta center above the profile, while a negative value indicates the profileis turning about a center below the profile. A zero indicates aninflection point where the profile is not curved. Arrows 561 and 562indicate the locations of projection points 531 and 532, respectively.

[0103] Both projection points are located near points where curve 560crosses zero. In this embodiment, dissection points are added whereverthe second derivative is interpolated to cross zero. This method can beused even for corner segments if the crossing of zero closest to thecorner is ignored. Using this method, one corner segment extends from 0nm to the point indicated by arrow 565, at about 220 nm, and a secondcorner extends from the point at arrow 568, at about 990 nm to 1200 nm.These lengths of 220 nm and 210 nm, respectively, are sent to a processto derive a corner dissection segment length, Lcor.

[0104] This technique also produces dissection points as indicated byarrows 563 at about 320 nm, 561 (associated with projection point 531)at about 480 nm, 566 at about 760 nm, and 564 at about 880 nm. Thesepoints yield non-corner segments of lengths 100 nm, 160 nm, 280 nm, 120nm, and 110 nm. These lengths are sent to the process for derivingnon-corner dissection parameters, and support a detail dissectionsegment length Ldet of about 110 nm, and a maximum dissection segmentlength Lmax of about 300 nm.

[0105] According to one embodiment, an evaluation point is placed at thepoint on each segment where the absolute value of the second derivativeis a maximum. For example, evaluation points are placed at 100 nm, 300nm, 400 nm, 800 nm, 950 nm, and 900 nm, where the second derivativecurve 560 reaches local extreme values between locations where the curve560 crosses zero.

[0106] Using these example embodiments, a proximity effects model is runfor a variety of locations along one or more edges of one or morepolygons. The polygons can be selected from a proposed or final masklayout, or from a design layer of a design layout, or from a test maskused to build the proximity effects model. The profile of model outputalong the edge is used to select dissection points and evaluation pointsfor that particular edge. The resulting segment lengths for theparticular edge are used with segment lengths obtained for other edgesto derive target lengths to use as dissection parameters.

[0107] This technique allows the proximity effects model to be run at afew locations, i.e., along certain edges, rather that at every point ina layout. Certain important edges are dissected directly, and theremaining edges are dissected using the dissection parameters. Theproximity effects model is then run for all other edges only at theevaluation points. Thus a large number of model runs are avoided and thecomputation proceeds at a desirable rate. At the same time, theevaluation points are reasonably selected to give the necessary detailon all the edges.

[0108] Deriving Dissection Parameters from a Test Mask

[0109] The profiles such as 540 can be generated by any method known inthe art. In one embodiment, the test patterns used to build the modelare used. These test patterns already include a range of features atdifferent scales and model output computed on a relatively dense outputgrid. Thus with substantially no additional computations than thosealready made to build the proximity effects model in the first place,model amplitudes along various polygon edges are already available forderiving dissection parameters.

[0110] In another embodiment, the corner length scale is derived fromthe model output at a series of features of varying pitch. Pitch refersto the smallest distance between repeated features in a layer. The useof features of varying pitch to derive a length scale appropriate for agiven model is shown in FIG. 6.

[0111]FIG. 6 shows an exemplary test pattern 692 of elongated polygonsfor a suite of fabrication equipment. FIG. 6 also shows a profilelocation 695, and the corresponding exemplary model 690 amplitudes alongthe profile. The pitch associated with portion 692 a of the test pattern692 is illustrated by the separation 691 a. Similarly, portions 692 b,692 c and 692 d have associated pitches 691 b, 691 c and 691 d,respectively, of ever increasing size. The section 695 is perpendicularto the elongated polygons. The profile 690 on the section shows aportion 690 a in which model output never exceeds the thresholdamplitude 470 associated with a feature being printed. Thus this portionof the test pattern is too closely spaced to be printed with theequipment suite modeled by the proximity effects model. Portion 690 b ofprofile 690 does cross the threshold amplitude, barely. Other portions690 c and 690 d cross substantially above the threshold. In thisexample, the particular pitch 691 b associated with profile portion 690b, represent a transition from unprintable to printable scales. Featurelengths smaller than this particular scale are not likely to havedifferent printable effects. Consequently, dissecting features intosegments smaller than this scale will likely generate too many segmentsand too many evaluation points. It is expected that corrections for suchsegments will not converge on a solution appropriate for the entirelayout. If profile portion 690 b had been entirely below the threshold,then a length scale representing the transition from unprintable toprintable scales is obtained by interpolating to a pitch at thethreshold amplitude on a plot of maximum amplitude versus pitch.

[0112] In one embodiment, the particular pitch length scale associatedwith maximum model output at the threshold value is used to set a cornerdissection segment length. In another embodiment, this particular pitchlength scale is used to set a detail dissection segment length.

[0113] Method for Deriving Dissection Parameters

[0114]FIG. 7 is a flow diagram for setting dissection parametersaccording to one embodiment of the present invention. This is oneembodiment of step 310 a for setting dissection parameters based on aproximity effects model output.

[0115] In step 710, a profile of proximity effects model amplitudes isgenerated along at least one edge of a polygon on a test layout. In thepreferred embodiment, the test pattern is one used to generate theproximity effects model. In another embodiment, the polygon can beselected on-the fly from a proposed fabrication layout for an actualprinted features layer.

[0116] In step 730, a corner dissection segment length parameter, Lcor,is set to a distance scale representing the distance from a corner,i.e., a vertex of the polygon defining the current edge, to a point onthe profile where the profile flattens. The profile is flat, forexample, where successive points undergo less than a certain change inamplitude or absolute value of the slope is less than a particularminimum value. In other embodiments, Lcor is set to a distance scalerepresenting the distance from the corner to a particular percentage ofthe flat portion, or a particular portion of the threshold, or where thesecond derivative crosses zero. In some embodiments, a set of Lcorparameters are defined, one for each corner type or angle range or both,and set to appropriate distance scales.

[0117] In step 750, dissection points are selected away from thecorners, defining non-corner segments, based on changes in model output,such as based on the first derivative or second derivative of the modelamplitude profile. This step can also be used to define a halo distance,i.e., a separation between features beyond which no dissection pointsare necessary on an edge. With a halo distance so defined, if somevertex of another edge is within the halo distance of a non-cornersegment of the edge whose profile is being examined, a dissection pointshould be necessary.

[0118] In step 770, the detail dissection segment length parameter,Ldet, is based on the segment lengths of the non-corner segments. Forexample, the detail dissection segment length is set equal to a mode inthe distribution of non-corner segment lengths associated with theshortest segment lengths. For a second example, the detail dissectionsegment length is set to a particular percentile of non-corner segmentlengths, such as the 5^(th) percentile length or 10^(th) percentilelength.

[0119] In step 790, the maximum dissection segment length parameter,Lmax, is based on the segment lengths of the non-corner segments thatare substantially longer than the detail dissection segment length. Forexample, the maximum dissection segment length is set equal to a secondmode in the distribution of non-corner segment lengths, a mode notassociated with the shortest segment lengths. For a second example, thedetail dissection segment length is set to a particular percentile ofnon-corner segment lengths, such as the 25^(th) percentile length, the50^(th) percentile length (the median segment length), or the 75^(th)percentile length.

[0120] When the halo and dissection lengths are defined, then controlpasses to step 330 to use these parameters to dissect polygons in anactual fabrication layout.

[0121] Modified Selection of Dissection and Evaluation Points

[0122] In one embodiment, dissection points are based on the vertices ofthe polygon and five prescribed dissection parameters. The fiveprescribed dissection parameters include:

[0123] corner segment dissection length (Lcor);

[0124] detail segment dissection length (Ldet);

[0125] maximum segment dissection length (Lmax);

[0126] rue-gate extension length (Lext); and

[0127] halo distance (halo).

[0128] The vertices and dissection parameters are first used todetermine an edge type. Edges shorter than the shortest prescribedsegment length are not worth evaluating and so have no dissection pointsor evaluation points selected. Edges long compared to twice the cornersegment length have a dissection point placed about Lcor from eachvertex. A projection point is located on an edge at the closest point toa vertex of another edge, if that vertex is within the halo distance.Long portions of edges away from the corners and without projectionpoints, i.e., residual edges, may be dissected to prevent segmentslonger than Lmax. Next to corner segments and near projection points,segments should be about Ldet long. Edges of true-gates have lengthsincreased by the true-gate extension length, Lext, to preventunderexposure of these important features.

[0129] In other embodiments, multiple versions of these parameters aredefined. For example several Lcor parameters are defined todistinguished convex from concave corners and to distinguish squarecorners from acute and middle angle corners. As another example, severalLext parameters are defined to distinguish extensions toward an end capfrom extensions toward a connector, explained below.

[0130] One use of prescribed dissection parameters Lcor and Ldet isillustrated in FIG. 8A. FIG. 8A is a plan view of a vertex 811 a of apolygon where two edges long compared to Lcor and Ldet meet. The vertex811 a is selected as a dissection point 830 a. On the horizontal edge,the next selected dissection point 830 b is a distance Lcor 820 from thevertex 811 a. Dissection points 830 a and 830 b define a corner segment.The first segment next to the corner segment has a length Ldet 822,which causes the dissection point 830 c to be selected. Similarly, onthe vertical edge, the first dissection point 830 a is the vertex 811 a,and the next selected dissection point 830 d is a distance Lcor 820 fromthe vertex 811 a, while the next dissection point 830 e is a distanceLdet from there.

[0131] There is one evaluation point 840 between each pair of dissectionpoints on an edge. Away from the corner segments, the evaluation pointsare placed substantially halfway between the dissection points. Forexample, evaluation point 840 b is halfway between its associateddissection points 830 b and 830 c. This is done because the evaluationpoint represents the whole segment between the dissection points; andany correction computed for the evaluation point, such as moving theedge x distance units outward, will be applied to the whole segment.Treating the whole segment together is reasonable because thecorrections needed are expected to vary slowly along a single segment.That is how the segment length parameters were chosen—to represent adistance associated with each substantial change in proximity effects.

[0132] In the corner segments of this embodiment, however, theevaluation points are not halfway. Instead, in this embodiment, theevaluation point is placed two-thirds (⅔) of the way from the convexvertex to the next dissection point. In other embodiments, theevaluation point is placed substantially more than halfway from thevertex to the next dissection point. The placement of evaluation pointsfarther from the vertex is reasonable because the proximity effects inthe corner tend to increase more rapidly as the corner is approached.Halfway from the corner the changes are still large compared to theinside dissection point which abuts the next segment. A smallercorrection is desired than at the halfway point to avoid big differenceswith the abutting interior segment.

[0133]FIG. 8A illustrates the use of two of the prescribed dissectionparameters. In addition to the segments with lengths substantially equalto the prescribed parameters, this embodiment includes some segmentswhich have lengths which depend on the prescribed parameters but varyfrom them by an amount determined by the actual length of an edgebetween successive vertices of the polygon. The derived lengths include

[0134] Effective line-end segment length (dmin);

[0135] Effective turn-end corner segment length (dcor); and

[0136] Effective interior, flat segment length (dmax).

[0137] Line-end segments occur on polygon edges that are no shorter thanthe shortest prescribed segment length but not long enough to generatemore than one segment. Line-ends are likely underexposed or overexposedalong their entire length. Such edges are not dissected further but dohave an evaluation point. Thus on such edges, the dissection points arethe vertices and the evaluation point is placed midway between. Thesegment and the edge are coincident. Though controlled by the prescribeddissection parameters, the actual segment lengths, dmin, are derivedfrom the vertex positions. FIG. 8B illustrates a line-end edge ofpolygon 810 b. Dissection points 830 f and 830 g are placed at verticesand evaluation point 840 e is placed midway between them. The segmenthas a length dmin 824 derived from the distance between the vertices.Because dmin is greater than the lesser of prescribed parameters Ldetand Lcor, but shorter than (Ldet+2*Lcor), this edge is treated as aline-end and is given segment length dmin substantially equal to itsedge length, L. Not shown is a line end comprising two concave vertices.It is treated the same way as the depicted line end with two convexvertices. An edge characterized as a line-end is never dissected intotwo segments.

[0138] Turn-end edges are also longer than the shortest prescribedlength but involve one concave vertex and one convex vertex, such asshown in FIG. 8C. Thus a turn-end often involves an underexposed corner811 d and an overexposed corner 811 e. In addition, a turn-end involvestwo corners so close together that there is not enough distance for asegment of length Ldet between them. That is, edge length L 826 is lessthan (Ldet+2*Lcor). Such an edge is unlikely to have a properly exposedsegment. Depending on the actual length of a turn-end edge, L 826, itgets either a single evaluation point at its midpoint with both verticesbeing dissection points, or it is split evenly by three dissectionpoints 830 h, 830 i and 830 k into two segments of length dcor 827, asshown in FIG. 8C. Though its segment length is controlled by theprescribed dissection parameters Lcor and Ldet, its actual segmentlength, dcor 827, is derived from the positions of the vertices andtheir edge length L 826. In the embodiment shown in FIG. 8C, theevaluation points 840 f and 840 g are placed in the middle of bothsegments. In another embodiment, at least one segment is considered acorner segment with an evaluation point more than halfway from a cornerto the middle dissection point 830 i.

[0139] Corner, line-end and turn-end dissection lengths are used inthose circumstances even if projection points are also present.Projection points control segment lengths in the interior portions of anedge away from the vertices, as will be described later below. Away fromcorner segments (including all of short edges like line-ends andturn-ends) and projection points, residual portions of an edge aredivided evenly so that no segment is greater than Lmax and no segment isshorter than the shorter of Lcor and Ldet. Though controlled by theprescribed dissection parameters Lcor, Ldet and Lmax, the actual lengthof these segments (dmax)(see dmax 829, for example, in FIG. 8D) isderived from the original vertex positions.

[0140] Line-end segments occur on polygon edges that are no shorter thanthe shortest prescribed segment length but not long enough to generatemore than one segment. Line-ends are likely underexposed or overexposedalong their entire length. Such edges are not dissected further but dohave an evaluation point. Thus on such edges, the dissection points arethe vertices and the evaluation point is placed midway between. Thesegment and the edge are coincident. Though controlled by the prescribeddissection parameters, the actual segment lengths, dmin, are derivedfrom the vertex positions. FIG. 8B illustrates a line-end edge ofpolygon 810 b. Dissection points 830 f and 830 g are placed at verticesand evaluation point 840 e is placed midway between them. The segmenthas a length dmin 824 derived from the distance between the vertices.Because dmin is greater than the lesser of prescribed parameters Ldetand Lcor, but shorter than (Ldet+2*Lcor ), this edge is treated as aline-end and is given segment length dmin substantially equal to itsedge length, L. Not shown is a line end comprising two concave vertices.It is treated the same way as the depicted line end with two convexvertices. An edge characterized as a line-end is never dissected intotwo segments.

[0141] Dissection due to a projection point is illustrated in FIG. 8E.In this embodiment, the halo distance 855 is used to determine whetherany vertex from another edge is within range to induce a sharp change inproximity effect on the current edge. For example, vertex 811 f ofpolygon 810 f is within halo distance 855 of the current edge of polygon810 e being dissected. A projection point is defined, as the closestpoint on the edge from a vertex of another edge within the halo distance855. In the preferred embodiment, the halo is defined based on analyzingmodel amplitudes along an edge on a test pattern as described above. Themodel radius is the greatest distance at which the model can display aneffect, e.g., the length scale of the longest kernel function used inthe model. The halo distance is found in this embodiment, by firstselecting a particular variation in proximity effect consideredsignificant. Then the halo distance is set substantially equal to thegreatest separation between an edge and a vertex of another edge thatexhibits the particular variation or more in the test patterns. Inanother embodiment, a distribution of observed variations is generated,and the particular significant variation is determined from a certainpercentile of these variations, e.g., the 10^(th) percentile. The halodistance can also be defined manually. In this embodiment, the halodistance is preferably defined to be less than or equal to the modelradius, e.g., by a pre-defined percentage less than 100% to multiply bythe model radius

[0142] In FIG. 8E, a line 853, parallel to the current edge of thepolygon 810 e and spaced the halo distance 855 apart, marks the area inwhich a vertex of another edge can cause a projection point. Sincevertex 811 f of another edge is within the halo distance, a projectionpoint 850 is found.

[0143] In the preferred embodiment, dissection in the vicinity of theprojection point is carried out by placing a dissection point 830 q atthe projection point 850 and at two other points 830 r and 830 s spacedLdet 822 from the projection point 850. Evaluation points 840 m and 840n are then placed at the midpoint of the segments so defined. In anotherembodiment, an evaluation point is placed at the projection point, andtwo dissection points spaced Ldet from each other straddle theevaluation point. Such an embodiment is not preferred because it doesnot allow for different corrections on either side of the projectionpoint as needed for most circumstances, for example, as needed for curve540 in FIG. 5B. In yet another embodiment, the segment centered on theprojection point of this previous embodiment is augmented with twoadditional segments of length Ldet, one each on either side of the firstsegment. While such an arrangement allows for different corrections nearto and on either side of the projection point, it requires threeevaluations rather than the two evaluations of the preferred embodiment.

[0144] Where more than one projection point are initially found on anedge within a distance of Ldet along the edge, only one projection pointis kept and used for dissection. Any technique that provides a singleprojection point from the multiple initial projection points can beused. According to this embodiment, the projection point kept is the onehaving the shortest distance to the vertex of another edge. After thisone is selected, all other projection points on the edge within Ldet ofthe kept projection point are discarded.

[0145] In step 910 of FIG. 9A the edge length L is determined from thevertices of the current edge. In step 912 it is determined whether theedge length is less than the minimum dissection length, e.g., the lesserof the corner dissection segment length Lcor and the detail dissectionsegment length Ldet. If so, the edge is too short to merit an evaluationpoint and the edge is discarded, i.e., not dissected and not given anevaluation point. Control then passes to step 390 of FIG. 3 to obtainthe next edge for dissection, if any.

[0146] In step 914 it is determined whether the edge length L, which isnecessarily greater than or equal to the minimum dissection length, isalso less than double the minimum dissection length. If so, then theedge is a line-end or turn end that is treated as a single segment.Dissection points are selected at the vertices in step 915. Control thencontrol passes to step 370 to select an evaluation point, e.g., in themiddle of the edge.

[0147] In step 916, the edge length L is necessarily greater than doublethe minimum dissection length, and it is determined whether the edge islong enough to accommodate two corner segments of length Lcor and anintervening segment of length Ldet. If not, the edge is treated as aline end or as a turn-end. In step 918 it is determined whether the edgeis a turn end, i.e., includes both a convex corner and a concave vertex.If not, the edge is a line end and dissection points are placed at thetwo vertices in step 915 and control passes to step 370 to place theevaluation point. If one vertex is a convex vertex and the other is aconcave vertex, so that the edge is a turn-end, it is determined in step920 whether the edge length L is less than double Ldet. If not, then theedge is split evenly into two segments with dissection points at bothvertices and at the mid-point of the edge in step 919. If the edgelength L is less than double Ldet, then the edge is kept as one segmentwith dissection points at the vertices in step 915. In either case,control then passes to step 370 to select evaluation points.

[0148] If it is determined in step 916 that the edge length is longenough, two corner segments are placed on the edge, by selecting bothvertices of the polygon as dissection points as well as selecting thetwo points spaced away from the two vertices by the distance Lcor instep 922. A residual edge length is computed by subtracting double thecorner dissection segment length, i.e., subtracting 2*Lcor, from theedge length L.

[0149] In step 930, one or more projection points are selected on theresidual edge, for example, as shown in FIG. 8E. If more than oneprojection point is found, only projection points spaced at least Ldetfrom another projection point are kept.

[0150] In step 940 of FIG. 9B, every pair of dissection points notinvolving a corner segment, such as dissection points related to aprojection point or segment adjacent to a corner segment, is examinedand a residual length LR of a further residual edge is computed as thedistance between the currently examined pair of points. When no moresuch pairs of points are left on the edge, flow control eventuallypasses to step 370 to select evaluation points.

[0151] In step 942 it is determined whether the residual length LR isless than twice Ldet. If so, the segment is not further dissected instep 943, and control passes to step 950 to examine the next such pairof dissection points, unless it is determined in step 951 that thecurrent pair is the last pair and control passes to step 370 in FIG. 3.Otherwise control passes to step 944.

[0152] In step 944 the residual length LR is necessarily greater than orequal to double Ldet, and it is determined whether the residual lengthLR is also less than triple Ldet. If not, the segment is split evenlyinto two segments by adding a dissection point in the midpoint of thesegment in step 945. If so, control passes to step 946.

[0153] In step 946 the residual length is necessarily greater than orequal to triple Ldet, and two dissection points are added, each Ldetfrom one of the current pair of dissection points being examined. Then anew residual edge length is computed for the remaining segment betweenthe two new dissection points. Control then passes to step 948.

[0154] In step 948, the new residual edge length is divided by Lmax, andthe ceiling integer N of the quotient is obtained. The effective lengthof the remaining segments, dmax, is computed as the residual lengthdivided by the ceiling integer N. By dividing the residual length by theceiling integer, dmax is guaranteed to be less than or equal to Lmax.Then N−1 evenly spaced dissection points, spaced apart by dmax, areselected in the remaining segment. Control then passes to step 951 toobtain the next such pair of dissection points in 950 unless the currentpair is the last pair and control passes to step 370 in FIG. 3.

[0155] In one embodiment dissection points and evaluation points arealso chosen to be at grid points of a predefined input grid. Thisadditional choice provides the advantage that previously computed modelruns can be used for analyzing differences after the edges getdissected.

[0156] Specifically Segmenting Certain Edges

[0157] In yet another embodiment, the proximity effects model is run forclosely spaced points along certain critical edges on a proposed layoutor layouts, as described above with respect to FIG. 5. Based on therates of change of model amplitude along the edge, such as the first andsecond derivative, the edge is divided into segments. For example, asegment is formed for every change in model amplitude along the edgeabove a predetermined minimum change. For another example, a segment isdefined between every inflection point where the second derivativecrosses zero. These segments are not derived from the dissectionparameters, but are directly observed on the certain edges. Dissectionpoints are then placed at the segment endpoints and an evaluation pointis placed between each pair of dissection points.

[0158] This embodiment offers the advantage of customizing thedissection for certain important edges and responding to the actualvariations in proximity effects observed along the particular edge.

[0159] However, this embodiment requires more computational resourcesthan using dissection parameters because the model is run at many newpoints along an edge in the fabrication layout for the actual design.This embodiment does not use the pre-computed model values created whenthe model was built.

[0160] Some computation time and power is saved compared to runningevaluations at all points on these edges, because a correction is onlycomputed at the more widely spaced evaluation points, one per segment.Once computed, the same correction is then applied to the whole segment.Additional computational time and power can be saved by performing theprocess only on certain edges expected to suffer from proximity effects,like those on critical features in the layout with dimensions close tothe resolution of the fabrication process, or where critical dimensionsare within a halo distance of another polygon. For example, this processis used only on edges that include an edge of a true-gate or an edge ofa true-gate interconnect.

[0161] Including Edges From Double Exposures

[0162] Some features on a printed layer are formed by exposing the samelayer with masks formed according to two fabrication layouts. Forexample, in a first exposure, narrow phase-shifted features are formed,then, in a second exposure, wider connections between phase-shiftedfeatures are formed. Polygons in the first exposure include pairs ofphase-shift windows called shifters, which pass light of the samewavelength but out of phase by 180 degrees (or π radians).

[0163]FIG. 10A illustrates two rectangular shifters 1010 a and 1010 bthat form a true-gate 1001 along a portion of the area between theshifters 1010. For example, shifter 1010 a is bounded by four edgesconnecting vertices 1031, 1036, 1037 and 1038. Also shown is a polygon1020 used in a second exposure to connect the true-gate 1001 to otherfeatures, not shown, with connecting features 1009 a and 1009 b. Edgesconnecting vertices 1040, 1043, 1045, 1046, 1047 and 1048 are on polygon1020. The polygon 1020 includes wide trim sections 1024 (i.e. 1024 a and1024 b) entirely within the shifters 1010 (i.e. 1010 a and 1010 b) wherethere is no material on the layer being exposed the second time. Thepurpose of the trim sections 1024 is to prevent overexposing the edge ofthe true-gate 1001 during the second exposure. The edges of the trimsections 1024 inside the shifters 1010 do not leave edges on the printedfeatures layer. For example, no edges are printed corresponding to maskedges connecting vertices 1043 to 1045, 1045 to 1046, and 1046 to 1047.Also, the edges of the shifters outside the trim polygon are notprinted. For example, the edge connecting vertex 1038 to vertex 1037 isnot printed.

[0164] Some edges have portions that are printed and portions that arenot printed. For example, no edge is printed between points 1043 and1042 of the polygon 1020 edge connecting points 1043 to 1040. Similarly,no edge is printed between points 1038 and 1042 of shifter 1010 a edgeconnecting point 1038 to point 1031. Such partially printed edgesusually involve a point that marks the intersection of a shifter edgewith a trim edge, like point 1042.

[0165]FIG. 10A also illustrates that some portions of some edges aremore important than other portions of the same edge, even though bothare printed. For example, on shifter edge connecting point 1031 to 1036,the portion making up an edge of the true-gate 1001, between points 1032and 1035, is more important than other portions of that edge, thatsimply provide an extension for the connecting features 1009 a and 1009b.

[0166] According to these embodiments, dissection points and evaluationpoints are not placed on edges that are not printed, such as outsideedges polygons for shifters or edges of trim polygons inside shifters.For example, when dissecting the polygon of shifter 1010 a, the edgeconnecting vertices 1031 and 1036 contains portions that will beprinted, therefore this edge is dissected. However, the edge connectingvertices 1037 to 1038 will not be printed; and therefore it is notdissected.

[0167] Also according to these embodiments, dissection points andevaluation points are moved or removed to ensure that segments do nothave lengths less than a minimum segment length, and that the evaluationpoints are on the physically important parts of the segments. Accordingto one embodiment, dissection points are initially placed at the cornersof the true-gate, i.e., at points 1032 and 1035 in addition to shiftervertexes 1031 and 1036. The edge of the true-gate is then dissected asif 1032 and 1035 were vertices of a polygon. For example, normally adissection point is placed at point 1032 and another dissection point isplaced a distance Lcor away at 1033, and the evaluation point 1034 isplaced two-thirds of the way from point 1032 to point 1033.

[0168] However, if the length of the resulting segment from vertex 1031to true-gate corner 1032 is less than the minimum segment length, e.g.,the lesser of Lcor and Ldet, a dissection point is not placed attrue-gate corner 1032. Instead, the dissection point at 1031 is used.The evaluation point 1034 for the segment remains on the true-gate edgetwo-thirds of the way between points 1032 and 1033. That is, theevaluation point is located as if the corner of the true-gate 1032 werea corner dissection point. Because the segment from vertex 1031 to point1032 is used to connect the corner of the shifter to the true-gate, itis referred to as the shifter extension, and the shifter extensionlength is the distance from shifter vertex 1031 to true gate vertex1032.

[0169] This technique has the advantage of simultaneously keeping allsegment lengths greater than or equal to the minimum segment length, butensuring that the evaluation points are on the true-gate, the criticalfeature in the layout. This is an advantage because it is desired thatthe correction determined at the evaluation point be applicable to thetrue-gate, which is the important feature. The dissection of an entireedge, like shifter polygon edge from vertex 1031 to 1036, consideringthe influence of true gate corners is implemented during an alternativeembodiment of step 330 of FIG. 3.

[0170] Where a portion of an edge is not printed, dissection points areselected to avoid segments shorter than the minimum dissection segmentlength and evaluation points are placed on the portion being printed.According to one embodiment, dissection points are initially placed atthe intersection of shifter and trim edges in addition to trim edgevertexes. For example, a dissection point is initially placed atintersection point 1042 and at point 1041, a distance Lcor away. Theevaluation point is then placed two-thirds of the way from point 1042 topoint 1041, as if the intersection point 1042 were a vertex. However, ifthe non-printed portion of the edge, from point 1043 to 1042, is shorterthan the lesser of Lcor and Ldet, the segment from 1041 to 1042 isextended to point 1043. That is, the segment is extended to the trim1024 a by the trim extension distance from 1042 to 1043. The evaluationpoint remains unchanged, at two-thirds the distance from theintersection point 1042 to the dissection point 1041. The remaining edgeof the polygon 1020 is then dissected according to the rules statedabove for non-corner segments.

[0171] This technique has the advantage of simultaneously keeping allsegment lengths greater than or equal to the minimum segment length, butensuring that the evaluation points are on the printed portion of theedge. This is an advantage because it is desired that the correctiondetermined at the evaluation point be applicable to the printed edge.

[0172] Other arrangements yield other results. For example, FIG. 10Bshows an arrangement that has one edge of true-gate 1002 formed by aportion of one edge of polygon 1021, between vertices 1054 f (of trimsection 1025 a) and 1054 g (of trim section 1025 b). The true gatecorners on this edge are at points 1055 a and 1055 b. Both of thesetrue-gate corners are also intersections of the shifters 1011 with thisedge of the polygon 1021. Whether this edge is further dissected dependson the distance between these corners. If the distance from 1055 a to1055 b is less than the minimum segment length, e.g., the lesser of Lcorand Ldet, then this edge of the polygon 1021, between vertices 1054 fand 1054 g, is not dissected; and no evaluation point is placed on thisedge.

[0173] If the distance between true-gate corners 1055 a and 1055 b isgreater than or equal to the minimum segment length, then the dissectionof this edge of polygon 1021 connecting vertices 1054 f to 1054 gproceeds as follows. Normally true-gate corners 1055 a and 1055 b aredissection points and the evaluation point 1056 is midway between thetwo, as if the true-gate corners 1055 a and 1055 b were polygon verticeson a line-end. However, if both edge portions from true-gate corner toedge vertex, i.e. edge portion from vertex 1054 f to true-gate corner1055 a and edge portion from true-gate corner 1055 b to vertex 1054 g,have lengths shorter than the minimum segment length, then thedissection points are moved to the vertices. The evaluation point 1056remains unchanged, midway between the corners of the true-gate. If onlyone of these edge portions from true-gate corner to vertex is less thanthe minimum, the dissection point is moved to the vertex for the shortedge portion. For example, if only the edge portion from vertex 1054 fto true-gate corner 1055 a has a length less than the minimum dissectionlength, then the dissection point is moved from 1055 a to 1054 f, whiletrue-gate corner 1055 b remains a dissection point. The evaluation pointis moved so that it is placed two-thirds of the way from true-gatecorner 1055 a to 1055 b.

[0174] These techniques illustrated with respect to FIG. 10B offer theadvantages of producing no segments that are too short, and producingevaluation points at locations of physical significance even on edges ofa polygon that are only partially printed.

[0175] Other edges of polygon 1021, such as those connecting vertices1054 b to 1054 c, 1054 c to 1054 d, 1054 d to 1054 e, and 1054 e to 1054f are inside shifter 1011 a and are not printed. Also edges of shiftersoutside the trim, such as edges connecting vertices 1053 c to 1053 d,1053 d to 1053 e, 1053 e to 1053 f, and 1053 f to 1053 a are notprinted. Therefore these edges of polygon 1021 and shifter 1011 are notdissected. Still other edges, such as that edge of polygon 1021connecting vertex 1054 a to 1054 b, are partially printed and arehandled as described above with respect to FIG. 10A. The edge connectingvertices 1053 a and 1053 b of shifter 1011 a form corners of thetrue-gate 1002 as described above with respect to FIG. 10A.

[0176] In order to print a true-gate area more aggressively, to ensureit will not be underexposed, a true-gate is often artificially extended.The amount that a true-gate is extended during design of a fabricationlayout, such as a mask layout, can be specified by a designer with atrue-gate extension length parameter Lext. In some instances, thedesigner may specify several true-gate extension length parameters fordifferent circumstances. For example, a designer may specify a firsttrue-gate extension for use where a true-gate is connected to anotherelement and a second true-gate extension for use where a true-gate iscapped by an end-cap.

[0177] The actual true-gate extension depends both on the parametersLext and the actual feature geometries. For example, with respect toFIG. 10A, if the distance from true-gate corner 1032 to shifter vertex1031 is less than Lext, then the true-gate 1001 is extended to theshifter vertex 1031. If, on the other hand, the distance from true-gatecorner 1035 to shifter vertex 1036 is greater than Lext, then thetrue-gate is extended only to point 1039, a distance equal to Lext fromthe original true-gate corner 1036. According to this example, the truegate 1001 originally lying from point 1032 to point 1035 has beenextended on both ends to lie from point 1031 to point 1039.

[0178] According to this embodiment, the dissection points are selectedto be consistent with the true-gate extension scheme employed in thefabrication layout. For example, the dissection of an edge with a truegate described above is begun after the true-gate is extended to havecorners at points 1031 and 1039.

[0179]FIG. 10C shows shifters 1012 shared across two true-gates 1003that are not connected to each other. For each true-gate, a polygon 1022provides a connector 1009 to other elements, not shown, trim regions1026, and an end cap 1090. For example, polygon 1022 a providesconnector 1009 d, trim regions 1026 a and 1026 b, and end cap 1090 a,whereas polygon 1022 b provides connector 1009 f, trim regions 1026 cand 1026 d, and end cap 1090 b.

[0180] If Lext parameters have not been defined, then dissectionproceeds as follows. Normally, dissection points are placed at thecorners of the true-gate. For example, dissections points are placed at1065 a and 1065 b for true-gate 1003 a, and at 1065 c and 1065 d fortrue-gate 1003 b. In addition, dissection points are placed a distanceLcor from the corner points of the true-gates. For example, a dissectionpoint is placed at 1063 b, a distance Lcor from true-gate corner 1065 b,for true-gate 1003 a. Similarly, a dissection point is placed at 1063 f,a distance Lcor from true-gate corner 1065 c, for true-gate 1003 b. Theevaluation point is placed two-thirds of the way along the resultingsegment from the true-gate corner. For example, evaluation point 1066 ais placed two-thirds of the way from true-gate corner 1065 b todissection point 1063 b. Similarly, evaluation point 1066 b is placedtwo-thirds of the way from true-gate corner 1065 c to dissection point1063 f. No evaluation point is placed on the edge portion between thetwo true-gates, from point 1065 b to point 1065 c, because that edge isnot printed and does not require correction.

[0181] However, if the distance between the true gates is less than theminimum segment length, e.g., the lesser of Lcor and Ldet, then thedissection points are moved to the middle point of the gap. For example,if the distance from true-gate corners 1065 b of true-gate 1003 a totrue-gate corner 1065 c of true-gate 1003 b is less than the minimum ofLcor and Ldet, then the dissection points at 1065 b and 1065 c are bothmoved to point 1063 d. The evaluation points are not moved. They remainas if the corners of the true-gate were vertices of the polygon.

[0182] If a Lext parameter has been defined, then dissection proceeds asfollows. True-gate 1003 a is extended to point 1063 c, a distance Lextfor end caps from the original true-gate corner at 1065 b. Similarly,true-gate 1003 b is extended to point 1063 e, a distance Lext for endcaps from the original true-gate corner at 1065 c. No evaluation pointis placed on the edge portion between the two true-gates, from point1063 c to point 1063 e, because that edge is not printed and does notrequire correction. Similarly the other corners of the true-gates, 1065a and 1065 d are moved outward a distance Lext for connectors to points1067 a and 1067 b, respectively. In some embodiments, Lext for end capsis equal to Lext for connectors; in other embodiments they havedifferent values. The evaluation points are placed two-thirds of the wayfrom the extended corners 1063 c and 1063 e, respectively, to thedissection points 1063 b and 1063 f, respectively, each dissection pointspaced a distance Lcor from the extended corners.

[0183] However, if the distance between the extended true-gates, frompoint 1063 c to point 1063 e, is less than the minimum dissectionlength, then the dissection points are not placed at 1063 c and 1063 e,but are both moved to the center point 1063 d. The evaluation points arenot moved.

[0184] The techniques described with respect to FIG. 10C have theadvantage of defining segments each greater than the minimum segmentlength for all segments including edges of important features liketrue-gates. If the entire shared shifter edge is printed, thesetechniques are implemented during an alternative embodiment of step 330of FIG. 3.

[0185] Functional Overview of Dissecting Doubly Exposed Edges

[0186]FIG. 11 illustrates a method for finishing one edge and selectinganother edge of a polygon that takes into account whether the edge or aportion of the edge is printed or not. These steps are included in analternate embodiment 392 a of step 392 shown in FIG. 3.

[0187] In step 1110 it is determined whether all of an edge is importantfor the edge on which dissection points and evaluation points were mostrecently selected. If all of the edge just completed is important, thencontrol passes to step 1120 where the next edge on the polygon is madethe current edge. If some of the edge is less important, such as becausesome of the edge is not a true-gate or some is not printed, then flowpasses to step 1115 in which additional dissection points areconsidered, such as at intersections of polygon edges with shifteredges, and at corners of true gates. Evaluation points are only definedon important portions of segments. After these steps are performed, thenflow passes to step 1120 to make the next edge the current edge.

[0188] In step 1125 it is determined whether any of the next edge isprinted. If not, flow does not return to step 330 (of FIG. 3) to selectdissection points. Instead, flow passes to step 390 of FIG. 3 todetermine whether this is the last edge on the current polygon.

[0189] If at least some of the edge is printed, flow goes eventually tostep 330 to select those dissection points on the next edge.

[0190] Computing Corrections for Evaluation Points

[0191] Once the dissection points and associated evaluation points areselected, the model is run for the actual layout only for the evaluationpoints, as shown by process 232 in FIG. 2. The resulting proximityeffects model amplitude is used to compute a correction distance duringprocess 234 of FIG. 2. The correction distance is then applied to theentire segment between dissection points, moving the segment out or inand creating a new polygon for an adjusted mask layout, as shown byprocess 236 in FIG. 2.

[0192] Iterative Use of Proximity Effects Model

[0193] Once the segments have been moved by the correction distance forall the affected polygons, the adjusted fabrication layout (produced byprocess 236 in FIG. 2) is completed. This is used as the finalfabrication layout 235 in one embodiment. The corrections can becombined with the original polygons to create new flat files ofpolygons. In the preferred embodiment, a correction is stored as anhierarchical subunit in the hierarchical representation of the originalpolygon in the initial fabrication layout.

[0194] In another embodiment, the adjusted fabrication layout is testedagain with the proximity effects model to determine whether theagreement between the printed features layer (249 in FIG. 2) and thedesign layout (225 in FIG. 2) satisfy a pre-defined specificationtolerance. If so, the corrections end. Otherwise, another round ofcorrections is performed based on the current correction to furtherimprove the agreement. This process iterates until the agreementconverges on the specified tolerance for all edges, or it is determinedthat further improvement is not possible. In this embodiment, therecently adjusted fabrication layout produced during process 236 becomesthe next proposed fabrication layout (231) and the processes 260 through236 are repeated. For example, dissection points and evaluation pointsare selected for the new proposed layout, printed features are predictedwith the model, and the differences are analyzed.

[0195] In one embodiment, only the new edges of the polygon generated bymoving certain segments by the corresponding correction distances aredissected. In this embodiment, the halo used to find projection pointsis also reduced for the next round of evaluations, so that fewercorrections are attempted each round, and the solution can converge.This is accomplished, for example, as shown in FIG. 11. FIG. 11 showsstep 1130, which determines whether the next edge to be processed is anew edge introduced since the last proximity effects model run. An edgewill only be new if it includes a segment that was moved as a result ofa correction. In step 1130, if the edge is not new, i.e., a segment wasnot moved, then the edge is skipped. Only new edges have dissectionpoints and evaluation points selected in step 330 of FIG. 3.

[0196] Hardware Overview

[0197]FIG. 12 is a block diagram that illustrates a computer system 1200upon which an embodiment of the invention is implemented. Computersystem 1200 includes a bus 1202 or other communication mechanism forcommunicating information, and a processor 1204 coupled with bus 1202for processing information. Computer system 1200 also includes a mainmemory 1206, such as a random access memory (RAM) or other dynamicstorage device, coupled to bus 1202 for storing information andinstructions to be executed by processor 1204. Main memory 1206 also maybe used for storing temporary variables or other intermediateinformation during execution of instructions to be executed by processor1204. Computer system 1200 further includes a read only memory (ROM)1208 or other static storage device coupled to bus 1202 for storingstatic information and instructions for processor 1204. A storage device1210, such as a magnetic disk or optical disk, is provided and coupledto bus 1202 for storing information and instructions.

[0198] Computer system 1200 may be coupled via bus 1202 to a display1212, such as a cathode ray tube (CRT), for displaying information to acomputer user. An input device 1214, including alphanumeric and otherkeys, is coupled to bus 1202 for communicating information and commandselections to processor 1204. Another type of user input device iscursor control 1216, such as a mouse, a trackball, or cursor directionkeys for communicating direction information and command selections toprocessor 1204 and for controlling cursor movement on display 1212. Thisinput device typically has two degrees of freedom in two axes, a firstaxis (e.g., x) and a second axis (e.g., y), that allows the device tospecify positions in a plane.

[0199] The invention is related to the use of computer system 1200 forproducing design layouts and fabrication layouts. According to oneembodiment of the invention, dissection points and evaluation points forfabrication layouts are provided by computer system 1200 in response toprocessor 1204 executing, e.g., as threads, one or more sequences of oneor more instructions contained in main memory 1206. For example, theevaluation point determining process runs as a thread 1252 on processor1204 based on evaluation point determining process instructions 1251stored in main memory 1206. Such instructions may be read into mainmemory 1206 from another computer-readable medium, such as storagedevice 1210. Execution of the sequences of instructions contained inmain memory 1206 causes processor 1204 to perform the process stepsdescribed herein. In alternative embodiments, hard-wired circuitry maybe used in place of or in combination with software instructions toimplement the invention. Thus, embodiments of the invention are notlimited to any specific combination of hardware circuitry and software.

[0200] The term “computer-readable medium” as used herein refers to anymedium that participates in providing instructions to processor 1204 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission media.Non-volatile media includes, for example, optical or magnetic disks,such as storage device 1210. Volatile media includes dynamic memory,such as main memory 1206. Transmission media includes coaxial cables,copper wire and fiber optics, including the wires that comprise bus1202. Transmission media can also take the form of acoustic or lightwaves, such as those generated during radio-wave and infra-red datacommunications.

[0201] Common forms of computer-readable media include, for example, afloppy disk, a flexible disk, hard disk, magnetic tape, or any othermagnetic medium, a CD-ROM, any other optical medium, punchcards,papertape, any other physical medium with patterns of holes, a RAM, aPROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, acarrier wave as described hereinafter, or any other medium from which acomputer can read.

[0202] Various forms of computer readable media may be involved incarrying one or more sequences of one or more instructions to processor1204 for execution. For example, the instructions may initially becarried on a magnetic disk of a remote computer. The remote computer canload the instructions into its dynamic memory and send the instructionsover a telephone line using a modem. A modem local to computer system1200 can receive the data on the telephone line and use an infra-redtransmitter to convert the data to an infra-red signal. An infra-reddetector can receive the data carried in the infra-red signal andappropriate circuitry can place the data on bus 1202. Bus 1202 carriesthe data to main memory 1206, from which processor 1204 retrieves andexecutes the instructions. The instructions received by main memory 1206may optionally be stored on storage device 1210 either before or afterexecution by processor 1204.

[0203] Computer system 1200 also includes a communication interface 1218coupled to bus 1202. Communication interface 1218 provides a two-waydata communication coupling to a network link 1220 that is connected toa local network 1222. For example, communication interface 1218 may bean integrated services digital network (ISDN) card or a modem to providea data communication connection to a corresponding type of telephoneline. As another example, communication interface 1218 may be a localarea network (LAN) card to provide a data communication connection to acompatible LAN. Wireless links may also be implemented. In any suchimplementation, communication interface 1218 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

[0204] Network link 1220 typically provides data communication throughone or more networks to other data devices. For example, network link1220 may provide a connection through local network 1222 to a hostcomputer 1224. Local network 1222 uses electrical, electromagnetic oroptical signals that carry digital data streams. The signals through thevarious networks and the signals on network link 1220 and throughcommunication interface 1218, which carry the digital data to and fromcomputer system 1200, are exemplary forms of carrier waves transportingthe information.

[0205] Computer system 1200 can send messages and receive data,including program code, through the network(s), network link 1220 andcommunication interface 1218.

[0206] The received code may be executed by processor 1204 as it isreceived, and/or stored in storage device 1210, or other non-volatilestorage for later execution. In this manner, computer system 1200 mayobtain application code in the form of a carrier wave.

[0207] In the foregoing specification, the invention has been describedwith reference to specific embodiments thereof. It will, however, beevident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention.The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. A method of correcting for proximity effectsassociated with an edge in a first fabrication layout corresponding to adesign layer, the design layer indicating target edges for a printedfeatures layer, the method comprising: determining whether a first partof the edge is less important in the design layer than a second part ofthe edge; if it is determined that the first part is less important thanthe second part, then establishing an evaluation point on the secondpart, and establishing no evaluation point on the first part,determining how to correct at least a portion of the edge for proximityeffects based on an analysis at the evaluation point.
 2. The method ofclaim 1, wherein the first part is determined to be less important thanthe second part if the first part does not correspond to a target edgein the design layer and the second part does correspond to a target edgein the design layer.
 3. The method of claim 1, wherein the first part isdetermined to be less important than the second part if the first partdoes not correspond to a true-gate in the design layer for an integratedcircuit and the second part does correspond to a true-gate in the designlayer.
 4. The method of claim 1, further comprising, if the second partcorresponds to an edge of a true-gate and a true-gate extensionparameter is defined, then, before establishing an evaluation point,extending the second part to displace the first part by a lesser of alength of the first part and the true-gate extension parameter.
 5. Themethod of claim 1, further comprising splitting the edge into segmentssuch that no segment on the edge has a length less than a minimum targetlength.
 6. A method of correcting for proximity effects associated withan edge in a first fabrication layout corresponding to a design layer,the design layer indicating target edges for a printed features layer,the method comprising: determining whether a first part of the edge isless important in the design layer than a second part of the edge; if itis determined that the first part is less important than the secondpart, then establishing a first initial segment on the first partextending from a junction of the first part and the second part to anopposite end of the first part, establishing a second initial segment onthe second part extending from the junction to an opposite end of thesecond part, and establishing an evaluation point on the second part;and determining how to correct at least a portion of the edge forproximity effects based on an analysis at the evaluation point.
 7. Themethod of claim 6, wherein: the method further comprises splitting thesecond initial segment into at least one corner segment extending fromthe junction for a distance substantially equal to a first target lengthfor corner segments; and said establishing the evaluation point furthercomprises placing the evaluation point on the corner segment.
 8. Themethod of claim 6, further comprising adjusting at least one of thefirst initial segment and the second initial segment so that no segmenton the edge has a length less than a minimum target length.
 9. Themethod of claim 6, further comprising establishing no evaluation pointon the first part.
 10. The method of claim 6, further comprising, if itis determined that the first part is less important than the secondpart, then: determining whether the first initial segment is less then aminimum target length; and if the first initial segment is less than theminimum target length, then discarding the first initial segment, andestablishing an adjusted segment by extending a corner segment on thesecond part onto the first part, wherein the corner segment on thesecond part extends from the junction.
 11. The method of claim 10, saidestablishing the adjusted segment further comprising, if the oppositeend of the first part is a vertex of the edge, then extending the cornersegment to the vertex.
 12. The method of claim 10, said establishing theadjusted segment further comprising, if the opposite end of the firstpart is not a vertex of the edge, then extending the corner segment to amidpoint of the first part.
 13. A method of using a computer to correctfor proximity effects associated with an edge in a first fabricationlayout corresponding to a design layer, the design layer indicatingtarget edges for a printed features layer, the computer having aprocessor coupled to a computer-readable medium, the computer-readablemedium storing at least a portion of the design layout, the printedfeatures layer corresponding to a portion of an integrated circuit, themethod comprising; determining whether a first part of the edge is lessimportant in the design layer than a second part of the edge; if it isdetermined that the first part is less important than the second part,then establishing an evaluation point on the second part, andestablishing no evaluation point on the first part, determining how tocorrect at least a portion of the edge for proximity effects based on ananalysis at the evaluation point.
 14. A method of using a computer tocorrect for proximity effects associated with an edge in a firstfabrication layout corresponding to a design layer, the design layerindicating target edges for a printed features layer, the computerhaving a processor coupled to a computer-readable medium, thecomputer-readable medium storing at least a portion of the designlayout, the printed features layer corresponding to a portion of anintegrated circuit, the method comprising; determining whether a firstpart of the edge is less important in the design layer than a secondpart of the edge; if it is determined that the first part is lessimportant than the second part, then establishing a first initialsegment on the first part extending from a junction of the first partand the second part, establishing a second initial segment on the secondpart extending from the junction, and establishing an evaluation pointon the second initial segment; and determining how to correct at least aportion of the edge for proximity effects based on an analysis at theevaluation point.
 15. A computer readable medium for correctingproximity effects associated with an edge in a first fabrication layoutcorresponding to a design layer, the design layer indicating targetedges for a printed features layer, the computer readable mediumcarrying instructions to cause one or more processors to perform:determining whether a first part of the edge is less important in thedesign layer than a second part of the edge; if it is determined thatthe first part is less important than the second part, then establishingan evaluation point on the second part, and establishing no evaluationpoint on the first part, determining how to correct at least a portionof the edge for proximity effects based on an analysis at the evaluationpoint.
 16. A computer readable medium for correcting proximity effectsassociated with an edge in a first fabrication layout corresponding to adesign layer, the design layer indicating target edges for a printedfeatures layer, the computer readable medium carrying instructions tocause one or more processors to perform: determining whether a firstpart of the edge is less important in the design layer than a secondpart of the edge; if it is determined that the first part is lessimportant than the second part, then establishing a first initialsegment on the first part extending from a junction of the first partand the second part to an opposite end of the first part, establishing asecond initial segment on the second part extending from the junction toan opposite end of the second part, and establishing an evaluation pointon the second part; and determining how to correct at least a portion ofthe edge for proximity effects based on an analysis at the evaluationpoint.
 17. A carrier wave for correcting proximity effects associatedwith an edge in a first fabrication layout corresponding to a designlayer, the design layer indicating target edges for a printed featureslayer, the carrier wave carrying instructions to cause one or moreprocessors to perform: determining whether a first part of the edge isless important in the design layer than a second part of the edge; if itis determined that the first part is less important than the secondpart, then establishing an evaluation point on the second part, andestablishing no evaluation point on the first part, determining how tocorrect at least a portion of the edge for proximity effects based on ananalysis at the evaluation point.
 18. A computer readable medium forcorrecting proximity effects associated with an edge in a firstfabrication layout corresponding to a design layer, the design layerindicating target edges for a printed features layer, the carrier wavecarrying instructions to cause one or more processors to perform:determining whether a first part of the edge is less important in thedesign layer than a second part of the edge; if it is determined thatthe first part is less important than the second part, then establishinga first initial segment on the first part extending from a junction ofthe first part and the second part to an opposite end of the first part,establishing a second initial segment on the second part extending fromthe junction to an opposite end of the second part, and establishing anevaluation point on the second part; and determining how to correct atleast a portion of the edge for proximity effects based on an analysisat the evaluation point.
 19. A computer system for correcting proximityeffects associated with an edge in a first fabrication layoutcorresponding to a design layer, the design layer indicating targetedges for a printed features layer, the printed features layercorresponding to a portion of an integrated circuit, the computer systemcomprising: a computer readable medium carrying data representing theedge and at least a portion of the design layout; and one or moreprocessors coupled to the computer readable medium, the one or moreprocessors configured for determining whether any portion of the edgecorresponds to a target edge in the design layer, if it is determinedthat a portion of the edge corresponds to a target edge, thendetermining whether to establish an evaluation point on the edge,executing a routine implementing a proximity effects model for theevaluation point to produce a model amplitude, determining a correctionbased on an analysis of the model amplitude, and applying the correctionto at least some of the edge.
 20. A computer system for correctingproximity effects associated with an edge in a first fabrication layoutcorresponding to a design layer, the design layer indicating targetedges for a printed features layer, the printed features layercorresponding to a portion of an integrated circuit, the computer systemcomprising: a computer readable medium carrying data representing theedge and at least a portion of the design layout; and one or moreprocessors coupled to the computer readable medium, the one or moreprocessors configured for determining whether a first part of the edgeis less important in the design layer than a second part of the edge; ifit is determined that the first part is less important than the secondpart, then establishing an evaluation point on the second part, andestablishing no evaluation point on the first part, executing a routineimplementing a proximity effects model for the evaluation point toproduce a model amplitude for the evaluation point, determining acorrection based on an analysis of the model amplitudes, and applyingthe corrections to at least some of the edge.
 21. A computer system forcorrecting proximity effects associated with an edge in a firstfabrication layout corresponding to a design layer, the design layerindicating target edges for a printed features layer, the printedfeatures layer corresponding to a portion of an integrated circuit, thecomputer system comprising: a computer readable medium carrying datarepresenting the edge and at least a portion of the design layout; andone or more processors coupled to the computer readable medium, the oneor more processors configured for determining whether a first part ofthe edge is less important in the design layer than a second part of theedge; if it is determined that the first part is less important than thesecond part, then establishing a first initial segment on the first partextending from a junction of the first part and the second part to anopposite end of the first part, establishing a second initial segment onthe second part extending from the junction to an opposite end of thesecond part, and establishing an evaluation point on the second part;and executing a routine implementing a proximity effects model for theevaluation point to produce a model amplitude for the evaluation point,determining a correction based on an analysis of the model amplitudes,and applying the corrections to at least some of the edge.
 22. A systemfor correcting proximity effects associated with an edge in a firstfabrication layout corresponding to a design layer, the design layerindicating target edges for a printed features layer, the systemcomprising: a means for determining whether any portion of the edgecorresponds to a target edge in the design layer and if it is determinedthat a portion of the edge corresponds to a target edge, thendetermining whether to establish an evaluation point on the edge; aproximity effects model for producing a model amplitude at theevaluation point; a means for analyzing the model amplitude to determinea correction at the evaluation point; and a means for applying thecorrection to at least some of the edge including the evaluation point.23. A system for correcting proximity effects associated with an edge ina first fabrication layout corresponding to a design layer, the designlayer indicating target edges for a printed features layer, the systemcomprising: a means for determining whether a first part of the edge isless important in the design layer than a second part of the edge; and ameans for establishing an evaluation point on the second part, andestablishing no evaluation point on the first part, if it is determinedthat the first part is less important than the second part; a proximityeffects model for producing a model amplitude at the evaluation point; ameans for analyzing the model amplitude to determine a correction at theevaluation point; and a means for applying the correction to at leastsome of the edge including the evaluation point.
 24. A system forcorrecting proximity effects associated with an edge in a firstfabrication layout corresponding to a design layer, the design layerindicating target edges for a printed features layer, the systemcomprising: a means for determining whether a first part of the edge isless important in the design layer than a second part of the edge; and ameans for establishing a first initial segment on the first partextending from a junction of the first part and the second part to anopposite end of the first part, and establishing a second initialsegment on the second part extending from the junction to an oppositeend of the second part, and establishing an evaluation point on thesecond part, if it is determined that the first part is less importantthan the second part; a proximity effects model for producing a modelamplitude at the evaluation point; a means for analyzing the modelamplitude to determine a correction at the evaluation point; and a meansfor applying the correction to at least some of the edge including theevaluation point.
 25. A method for fabricating a printed features layerincluding features corrected for proximity effects, the methodcomprising: building an initial fabrication layout based on a designlayout, the design layer indicating target edges for a printed featureslayer, the fabrication layout including an edge; determining whether anyportion of the edge corresponds to a target edge in the design layer; ifit is determined that a portion of the edge corresponds to a targetedge, then determining whether to establish an evaluation point on theedge, executing a routine implementing a proximity effects model forproducing a model amplitude at the evaluation point, analyzing the modelamplitude to determine a correction, and applying the correction to atleast part of the edge; producing a mask based on the edge with thecorrection; and producing the printed features layer in a fabricationprocess using the mask.
 26. A method for fabricating a printed featureslayer including features corrected for proximity effects, the methodcomprising: building an initial fabrication layout based on a designlayout, the design layer indicating target edges for a printed featureslayer, the fabrication layout including an edge; determining whether afirst part of the edge is less important in the design layer than asecond part of the edge; if it is determined that the first part is lessimportant than the second part, then establishing an evaluation point onthe second part, and establishing no evaluation point on the first part;executing a routine implementing a proximity effects model for producinga model amplitude at the evaluation point; analyzing the model amplitudeto determine a correction; applying the correction to at least some ofthe edge; producing a mask based on the edge with the correction; andproducing the printed features layer in a fabrication process using themask.
 27. A method for fabricating a printed features layer includingfeatures corrected for proximity effects, the method comprising:building an initial fabrication layout based on a design layout, thedesign layer indicating target edges for a printed features layer, thefabrication layout including an edge; determining whether a first partof the edge is less important in the design layer than a second part ofthe edge; if it is determined that the first part is less important thanthe second part, then establishing a first initial segment on the firstpart extending from a junction of the first part and the second part toan opposite end of the first part, establishing a second initial segmenton the second part extending from the junction to an opposite end of thesecond part, and establishing an evaluation point on the second part;executing a routine implementing a proximity effects model for producinga model amplitude at the evaluation point; analyzing the model amplitudeto determine a correction; applying the correction to at least some ofthe edge; producing a mask based on the edge with the correction; andproducing the printed features layer in a fabrication process using themask.
 28. A mask for fabricating a printed features layer, the maskincluding an opaque regions having edges according to a fabricationlayout, the fabrication layout corresponding to a design layer, thedesign layer indicating target edges for a printed features layer,wherein: at least one edge is displaced from a corresponding target edgein the design layer by a correction distance; the correction distance isbased on analysis of an amplitude output by a proximity effects model atan evaluation point on the corresponding target edge; and whether anedge has an evaluation point is determined based on determining whetherany portion of the edge corresponds to a target edge in the designlayer.
 29. A mask for fabricating a printed features layer, the maskincluding an opaque region having an edge corrected for proximityeffects, the edge corresponding to a target edge in a design layer, thedesign layer indicating target edges for a printed features layer,wherein: the edge is displaced from the corresponding target edge in thedesign layer by a correction distance; the correction distance is basedon analysis of an amplitude output by a proximity effects model at anevaluation point on the corresponding target edge; and the evaluationpoint is based on determining whether a first part of the edge is lessimportant in the design layer than a second part of the edge, and if itis determined that the first part is less important than the secondpart, then establishing an evaluation point on the second part, andestablishing no evaluation point on the first part.
 30. A mask forfabricating a printed features layer, the mask including an opaqueregion having an edge corrected for proximity effects, the edgecorresponding to a target edge in a design layer, the design layerindicating target edges for a printed features layer, wherein: the edgeis displaced from the corresponding target edge in the design layer by acorrection distance; the correction distance is based on analysis of anamplitude output by a proximity effects model at an evaluation point onthe corresponding target edge; and the evaluation point is based ondetermining whether a first part of the edge is less important in thedesign layer than a second part of the edge, and if it is determinedthat the first part is less important than the second part, thenestablishing a first initial segment on the first part extending from ajunction of the first part and the second part to an opposite end of thefirst part, establishing a second initial segment on the second partextending from the junction to an opposite end of the second part, andestablishing an evaluation point on the second part.
 31. A device havinga proximity-corrected element, the device produced using a maskincluding an opaque regions having edges according to a fabricationlayout, the fabrication layout corresponding to a design layer, thedesign layer indicating target edges for a printed features layer,wherein: at least one edge is displaced from a corresponding target edgein the design layer by a correction distance; the correction distance isbased on analysis of an amplitude output by a proximity effects model atan evaluation point on the corresponding target edge; and whether anedge has an evaluation point is determined based on determining whetherany portion of the edge corresponds to a target edge in the designlayer.
 32. A device having a proximity-corrected element, the deviceproduced using a mask including an opaque region having an edgecorrected for proximity effects, the edge corresponding to a target edgein a design layer, the design layer indicating target edges for aprinted features layer, wherein: the edge is displaced from thecorresponding target edge in the design layer by a correction distance;the correction distance is based on analysis of an amplitude output by aproximity effects model at an evaluation point on the correspondingtarget edge; and the evaluation point is based on determining whether afirst part of the edge is less important in the design layer than asecond part of the edge, and if it is determined that the first part isless important than the second part, then establishing an evaluationpoint on the second part, and establishing no evaluation point on thefirst part.
 33. A device having a proximity-corrected element, thedevice produced using a mask including an opaque region having an edgecorrected for proximity effects, the edge corresponding to a target edgein a design layer, the design layer indicating target edges for aprinted features layer, wherein: the edge is displaced from thecorresponding target edge in the design layer by a correction distance;the correction distance is based on analysis of an amplitude output by aproximity effects model at an evaluation point on the correspondingtarget edge; and the evaluation point is based on determining whether afirst part of the edge is less important in the design layer than asecond part of the edge, and if it is determined that the first part isless important than the second part, then establishing a first initialsegment on the first part extending from a junction of the first partand the second part to an opposite end of the first part, establishing asecond initial segment on the second part extending from the junction toan opposite end of the second part, and establishing an evaluation pointon the second part.